1. Field of the Invention
The present invention relates to a method for manufacturing a silicon carbide semiconductor device, in particular, a method for manufacturing a silicon carbide semiconductor device having a trench.
2. Description of the Background Art
Japanese Patent Laying-Open No. 7-326755 discloses a silicon carbide semiconductor device having a trench. This patent publication describes that a gate thermal oxidation film has a thicker film thickness on a bottom surface of the trench than that on a side surface of the trench, thereby achieving a low threshold voltage and a high breakdown voltage between the gate and the drain. Also described therein is as follows. That is, the bottom surface of the trench corresponds to a carbon plane of hexagonal single-crystal silicon carbide. The carbon plane allows for fast oxidation rate. The side surface of the trench corresponds to a plane perpendicular to this carbon plane and allowing for slow oxidation rate. Accordingly, by performing a thermal oxidation step once, a thermal oxidation film can be formed such that the thickness thereof on the side surface of the trench is greatly different from the thickness thereof on the bottom surface of the trench.
In a method that employs crystal orientation dependence of thermal oxidation rate such as the technique described in the above-described patent publication, there is a limit in increasing the thickness of the gate insulating film on the bottom portion of the trench as compared with the thickness thereof on the side wall of the trench. In addition, the crystal orientation dependence of the thermal oxidation rate becomes small unless a thermal oxidation temperature lower than that in a normal case is employed. For this reason, with this method, low threshold voltage and large breakdown voltage are less likely to be sufficiently attained at the same time.